Subject: Re: Security Random Number Generator support

From: Marc Zyngier <[email-masked]>
Date: Wed, 03 Jun 2020 08:40:58 +0100

On 2020-06-03 08:29, Neal Liu wrote:
> On Tue, 2020-06-02 at 21:02 +0800, Marc Zyngier wrote:
>> On 2020-06-02 13:14, Ard Biesheuvel wrote:
>> > On Tue, 2 Jun 2020 at 10:15, Neal Liu <[email-masked]> wrote:
>> >>
>> >> These patch series introduce a security random number generator
>> >> which provides a generic interface to get hardware rnd from Secure
>> >> state. The Secure state can be Arm Trusted Firmware(ATF), Trusted
>> >> Execution Environment(TEE), or even EL2 hypervisor.
>> >>
>> >> Patch #1..2 adds sec-rng kernel driver for Trustzone based SoCs.
>> >> For security awareness SoCs on ARMv8 with TrustZone enabled,
>> >> peripherals like entropy sources is not accessible from normal world
>> >> (linux) and rather accessible from secure world (HYP/ATF/TEE) only.
>> >> This driver aims to provide a generic interface to Arm Trusted
>> >> Firmware or Hypervisor rng service.
>> >>
>> >>
>> >> changes since v1:
>> >> - rename mt67xx-rng to mtk-sec-rng since all MediaTek ARMv8 SoCs can
>> >> reuse
>> >>   this driver.
>> >>   - refine coding style and unnecessary check.
>> >>
>> >>   changes since v2:
>> >>   - remove unused comments.
>> >>   - remove redundant variable.
>> >>
>> >>   changes since v3:
>> >>   - add dt-bindings for MediaTek rng with TrustZone enabled.
>> >>   - revise HWRNG SMC call fid.
>> >>
>> >>   changes since v4:
>> >>   - move bindings to the arm/firmware directory.
>> >>   - revise driver init flow to check more property.
>> >>
>> >>   changes since v5:
>> >>   - refactor to more generic security rng driver which
>> >>     is not platform specific.
>> >>
>> >> *** BLURB HERE ***
>> >>
>> >> Neal Liu (2):
>> >>   dt-bindings: rng: add bindings for sec-rng
>> >>   hwrng: add sec-rng driver
>> >>
>> >
>> > There is no reason to model a SMC call as a driver, and represent it
>> > via a DT node like this.
>> 
>> +1.
>> 
>> > It would be much better if this SMC interface is made truly generic,
>> > and wired into the arch_get_random() interface, which can be used much
>> > earlier.
>> 
>> Wasn't there a plan to standardize a SMC call to rule them all?
>> 
>>          M.
> 
> Could you give us a hint how to make this SMC interface more generic in
> addition to my approach?
> There is no (easy) way to get platform-independent SMC function ID,
> which is why we encode it into device tree, and provide a generic
> driver. In this way, different devices can be mapped and then get
> different function ID internally.

The idea is simply to have *one* single ID that caters for all
implementations, just like we did for PSCI at the time. This
requires ARM to edict a standard, which is what I was referring
to above.

There is zero benefit in having a platform-dependent ID. It just
pointlessly increases complexity, and means we cannot use the RNG
before the firmware tables are available (yes, we need it that
early).

         M.
-- 
Jazz is not dead. It just smells funny...

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